1. Field of the Invention
The present invention relates to an image display apparatus. In particular, the invention relates to an image display apparatus capable of producing a proper display image by using look-up tables in performing nonlinear processing such as a white balance adjustment and gamma correction in image display of displays, projectors, etc. as well as to a data generation method used in storing correction data in look-up tables.
2. Description of the Related Art
The video signal of an image of usual TV broadcast is transmitted after being subjected to xcex3 correction so as to be suitable for the current-luminance characteristic of the CRT. Therefore, in performing image display based on such a video signal by using a display apparatus other than the CRT, it is necessary to perform gradation correction (hereinafter referred to as xe2x80x9cxcex3 correctionxe2x80x9d) that is suitable for the electro-optical conversion characteristic of the display apparatus.
FIG. 1 is a four-quadrant diagram illustrating such xcex3 correction that is performed for a liquid crystal panel as a display apparatus. In the case of a TV video signal, as shown in part (a) of FIG. 1, a low-level portion and a high-level portion of the original video signal is expanded and compressed, respectively, according to a ({fraction (1/2.2)})th-power curve before transmission.
Gamma correction for a video signal needs to be determined in consideration of the V-T characteristic of the liquid crystal panel so that the relationship between the original signal and the luminance on the display screen becomes linear as shown in part (b) of FIG. 1 with intervention of the V-T characteristic of the liquid crystal panel shown in part (c) of FIG. 1. Part (d) of FIG. 1 shows an example of xcex3 correction characteristics.
A received video signal is subjected to such xcex3 correction and a corrected signal is used as a drive signal of the liquid crystal panel.
With the above xcex3 correction, signal processing can be performed so that the luminance of the liquid crystal panel properly reflects the level of an original video signal and hence the contrast of an original image can be reproduced correctly.
Also in the case of a color image, by performing such xcex3 correction for each of the three primary colors, the hue of an original image can be reproduced faithfully and color temperature setting and a white balance adjustment can be attained by adjusting xcex3 correction values.
In the case of the liquid crystal display panel, the xcex3 correction assumes a nonlinear, S-shaped curve, which can be approximated by using a polygonal line circuit in the case of an analog signal.
However, in recent years, for more faithful gradation expression, signal processing shown in FIG. 2, for example, has come to be employed in which digital signals of xcex3-corrected gradations are read out by supplying 8-bit digital color video signals of R (red), G (green), and B (blue) (in the case of analog input signals, they are digitized in advance by A/D converters) to look-up tables (memories) 11R, 11G, and 11B in which xcex3 correction data of the respective colors are stored in advance, the read-out digital signals are converted into analog signals by D/A converters 12R, 12G, and 12B, and the analog signals are supplied as drive signals to liquid crystal panels 14R, 14G, and 14B of the respective colors via liquid crystal driving circuits 13R, 13G, and 13B.
FIG. 3 shows an example of xcex3 correction performed by such a signal processing circuit. In FIG. 3, curves xcex3R, xcex3G, and xcex3B of xcex3 correction data that are stored in advance allow readout of output data (vertical axis) for gradation data of 8 bits (0-256) of video signals that are input to the look-up tables. As a result, for gradation levels of transmission-side xcex3-corrected color signals, this xcex3 correction enables output of corrected signals with intervention of V-T drive characteristic curves VTr, VTg, and VTb of the liquid crystal panels.
A white balance adjustment is performed finally. Where only amplitude and offset adjustments are performed by the digital signal processing of the look-up tables, to reproduce white by xcex3-corrected signals of the respective colors by compensating for optical variations of the liquid crystal panels, the ranges of signals are restricted. Therefore, unnecessary signal ranges of the respective colors are eliminated. As shown in FIG. 3, usable xcex3-corrected output signal ranges of R, G, and B after the white balance adjustment are 20-100%, 8-80%, and 0-95%, respectively.
That is, only parts, in ranges Ra, Ga, and Ba, of the xcex3 correction curve data stored in the look-up tables are effectively used as xcex3 correction data and the other data are not used.
In particular, in the case of the transmission-type liquid crystal panels, the contrast ratio of B is lower than the contrast ratios of the other colors. For example, in general, the contrast ratios of R and G on the liquid crystal panels are about 600:1 and about 500:1, for example. However, in the case of B, even in a case where the maximum drive voltage is applied, the light attenuation factor is low and there remains light that passes through the panel (leakage light). Therefore, the contrast ratio of B is as small as about 400:1. As a result, the screen becomes bluish in a black range.
In view of the above, it is necessary to obtain proper white balance in a black-level-elevated state by adding the other colors to blue of minimum luminance. If the dynamic ranges of the other colors are so set equal to the dynamic range of B, as shown in FIG. 3 the upper limit and the lower limit portions of drive voltages of R and G go out of the usable ranges after the white balance adjustment.
That is, the usable ranges after the white balance adjustment are defined by the maximum values and minimum values that provide a constant luminance ratio of R, G, and B that is R:G:B=22:68:10, for example. Upper limits are determined by a white-side luminance ratio of R, G, and B. Black-side usable ranges of the respective colors are determined by a black-side luminance ratio of R, G, and B, that is, (white-side luminance ratio)xc3x97{1/(contrast of each color)}.
As described above, in the conventional xcex3 correction method, not all of correction data of look-up tables that are stored in advance and conform to the V-T characteristics of the liquid crystal panels are used effectively; that is, the amount of data of the look-up tables that are used effectively is reduced. This results in a problem that the gradations of correction data unavoidably become coarse.
In the case of B, in particular, although the xcex3 characteristic varies steeply in a dark gradation range, output xcex3-corrected values in this range have coarse gradations. This means a problem that a white balance adjustment cannot be performed closely in this dark range.
The present invention has been made in view of the above problems. According to a first aspect of the invention, there is provided an image display apparatus comprising look-up tables in which xcex3 correction data for color signals are stored, D/A converters for converting color image data that are read out from the look-up tables into analog signals, respectively, driving means for generating drive signals based on the analog signals, and optical modulating means for displaying a color image based on the drive signals, wherein xcex3 correction data that attain proper white balance in dynamic ranges that can be used for display are stored in the look-up tables, and wherein the amplitudes and offsets of drive signals in such ranges as not to be corrected by the look-up tables are adjusted by signal transmission characteristics of an analog system.
According to a second aspect of the invention, there is provided a gradation correction data generation method for an image display apparatus comprising look-up tables in which xcex3 correction data for color signals are stored, D/A converters for converting color image data that are read out from the look-up tables into analog signals, respectively, driving means for generating drive signals based on the analog signals, and optical modulating means for displaying a color image based on the drive signals, the gradation correction data generation method comprising the steps of, in a white balance adjustment and a xcex3 adjustment, setting dynamic ranges that can be used for display by measuring a luminance ratio and contrast ratios for the color signals, and performing a gain adjustment and an offset adjustment after D/A conversion in such a manner that they are suitable for the dynamic ranges; and re-calculating new xcex3 correction data of the look-up tables so that the new xcex3 correction data have the dynamic ranges as full ranges, and substituting the new xcex3 correction data for the previous xcex3 correction data of the look-up tables, whereby xcex3 correction data within the dynamic ranges can be used effectively in the white balance adjustment that is performed on digital data.
As described above, in the invention, to make such settings that all the digital data stored in the look-up tables are used effectively in digitally performing such adjustments as xcex3 correction and a white balance adjustment by the look-up tables, offsets and dynamic ranges are compensated by subsequent analog circuits. Therefore, the adjustments can be performed by using finer luminance levels than in the conventional case even if memories having the same capacities as in the conventional case are used for storing xcex3 correction data.